Differences in melon-headed whale (Peponocephala electra) whistle characteristics between sympatric populations in Hawaii

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Melon-headed whales (Peponocephala electra) are found throughout the tropics, primarily in pelagic waters but with some being found near-shore around oceanic islands. This species is considered to be sensitive to sound, with one nearmass stranding event in Hawaiian waters associated with a naval exercise. However, relatively little is known of the acoustic behavior of this species. Here we combine multiple passive acoustic tools, including suction-cup attached acoustic tags (DTAGs) and a broadband recorder, the DMON, to gather baseline, broadband bioacoustic behavior data for melon headed whales. Further, two distinct melon-headed populations are compared: the Kohala resident population (KRP) and the Hawaiian Islands population (HIP). DMONs recorded from a towfish and drifter buoy setup. Three animals were tagged, two from the KRP population (estimated group sizes 280 and 350 ! individuals) and one from the HIP population (estimated group size of 350 individuals). In total, 1096 and 329 whistles have been identified from the KRP and HIP tag recordings, respectively. Analyses included spectral and temporal characteristics of the whistles, variability of these signals and whistle type categorizations. Data suggest variation between melon-headed whale populations in whistle duration (mean: KRP – 0.99 s; HIP – 0.75 s), start frequency (mean: KRP – 5.1 kHz; HIP – 5.8 kHz), end frequency (mean: KRP – 4.8 kHz; HIP – 11.5 kHz), peak frequency (mean: KRP – 5.5 kHz; HIP – 7.6 kHz), and centroid frequency (mean: KRP – 7.7 kHz; HIP – 10.1 kHz). No overlap in whistle type was apparent between or within populations, which suggests that individual animals may produce semi-stereotyped calls. These are the first broadband comparisons of melon-headed whale signals. The whistle characterization differences suggest automatic detection and classification algorithms can be used to identify melon-headed whales in Hawaiian waters.